1. Field of the Invention
The present invention relates to an optical semiconductor device and, more particularly, an optical semiconductor device used as an optical modulator in the optical transmitter in the optical communication system.
2. Description of the Prior Art
In the optical communication system, as the devices used as the optical modulator in the optical transmitter, there are devices having such a structure that has a function of modulating the continuous light based on the waveform of the input electric signal (voltage). Out of them, the device that deals with the very high-speed communication at a communication rate of 10 to 80 Gbits/sec or more is employed.
As the device having such function, the Mach-Zehnder optical modulator using LiNbO3 as the material can be listed. FIG. 1 is a schematic view showing such device viewed from the top.
The optical modulator shown in FIG. 1 has an interferometer including two optical waveguides 101, 102, two optical couplers 103, 104, and one optical phase modulator 105. Input ends of the first and second optical waveguides 101, 102 are connected to the first optical coupler 103 to separate the continuous light into two continuous lights. Also, Output ends of them are connected to the second optical coupler 104 to recombine two separated continuous lights. Thus, the modulated optical signal is generated.
The optical phase modulator 105 is comprised of a signal electrode 105a formed on the first optical waveguide 101, and first and second ground electrode 105b, 105c formed at an interval on both sides of the signal electrode 105a. One ends of the first and second ground electrode 105b, 105c are connected electrically to one end of the signal electrode 105a via a high-frequency electric signal source 106 respectively. Also, the other ends of the first and second ground electrode 105b, 105c are connected electrically to the other end of the signal electrode 105a via a terminating resistance 107 respectively.
A sectional structure of the optical phase modulator 105 in the optical modulator shown in FIG. 1, if viewed along a I—I line, is shown in FIG. 2.
A core 110a of the first optical waveguide 101 is formed by diffusing titanium into a surface layer of a LiNbO3 substrate 100. A core of the second optical waveguide 102, although not particularly shown, has the same structure as the core 101a of the first optical waveguide 101.
The LiNbO3 substrate 100 is covered with a buffer layer 108 made of dielectrics such as SiO2, or the like. The signal electrode 105a and the first and second ground electrodes 105b, 105c are formed on the buffer layer 108 by the gold (Au) plating.
In such structure, when a signal voltage is applied between the signal electrode 105a and the ground electrodes 105b, 105c from the high-frequency electric signal source 106, the electric field generated around the signal electrode 105a is distributed in areas between the signal electrode 105a and the ground electrodes 105b, 105c to have an almost uniform intensity. A part of the electric field is applied to the core 110a of the first optical waveguide 101 to cause change in the refractive index.
Normally, the cross section shown in FIG. 2 is the structure that is designed as the traveling-wave electrode structure. The reflection from the device, to which the high-frequency electric signal is applied, to the driver circuit is suppressed by matching the characteristic impedance into 50 Ω. Normally, a length of the device that uses LiNbO3-based material as the material of the modulator (optical modulator length) becomes very large like about 40 mm.
Meanwhile, the optical modulator using the semiconductor as the device material is also established. A length of the device using the semiconductor material becomes several mm at its maximum, and is smaller than the device using LiNbO3 as the material. Also, if the semiconductor is used as the substrate material, the optical modulator can be integrated together with the light source using the semiconductor as the material, e.g., the laser diode.
Schematic configurative views of the Mach-Zehnder optical modulator using the semiconductor as the material in the prior art are shown in FIG. 3 to FIG. 5. FIG. 4 is a sectional view taken along a II—II line in FIG. 3, and FIG. 5 is a sectional view taken along a III—III line in FIG. 3. Such optical modulator is set forth in 10th International Conference on Indium Phosphide and Related Materials WA3-4(1998), for example.
In such optical modulator, like the above optical modulator made of LiNbO3-based material, the traveling-wave electrode is formed in the portion into which the electric signal is input.
The optical modulator shown in FIG. 3 has two ridge-type optical waveguides 111, 112, two optical couplers 113, 114, and a traveling-wave electrode 115. A structure of the traveling-wave electrode 115 shown in FIG. 3 is called a capacitive-load/slot-line type electrode structure hereinafter.
The traveling-wave electrode 115 is roughly divided into two kinds of areas. One area out of these areas is shown as two slot-line electrodes 115a, 115b that are arranged at an interval on both sides of the optical waveguides 111, 112 and have a large width. A high-frequency electric signal source 116 is connected to one ends of the first slot-line electrode 115a and the second slot-line electrode 115b. The other ends of them are connected together via a terminating resistance 131.
These two slot-line electrodes 115a, 115b constitute the slot-line type transmission line if narrow electrode portions that are derived on the inside of them are omitted from consideration. This slot-line type transmission line functions as the transmission line that propagates the signal, which is supplied from the high-frequency electric signal source 116 connected to the device end, to the entire device.
The other area is phase modulation electrodes 115c, 115d of the phase modulator portion that are formed in an area between two slot-line electrodes 115a, 115b. The phase modulation electrodes 115c, 115d are formed over two ridge-type optical waveguides (arms) 111, 112 constituting the Mach-Zehnder interferometer, and are formed to apply the electric field to the core layer of the optical waveguide in the arms 111, 112.
The phase modulation electrodes 115c, 115d can be regarded electrically as the capacitance (electric capacitance), as described later. Then, the electric capacitances are connected to the slot-line electrodes 115a, 115b via the bridge wirings. Therefore, the overall structure of the traveling-wave electrode 115a, 115b shown in FIG. 3 is called the capacitive-load slot-line electrode structure.
The reason why the traveling-wave electrode 115 is divided into two areas in this manner is that the characteristic impedance is caused to coincide with a desired value by regarding the entirety of them as one high-frequency transmission line.
FIG. 4 is a II—II line sectional view of the phase modulator portion and its periphery in the semiconductor optical modulator shown in FIG. 3.
In FIG. 4, a high-resistance InP buffer layer 121, a first InGaAsP etch-stop layer 122, an n+-InP layer 123, an n−-InP layer 124, a MQW layer 125, a first i-InP layer 126, a second InGaAsP etch-stop layer 127, a second i-InP layer 128, a p-InP layer 129, and a p-InGaAsP contact layer 130 are formed sequentially on a high-resistance InP substrate 120. Then, respective layers from the first InGaAsP etch-stop layer 122 to the second InGaAsP etch-stop layer 127 are patterned into a shape that includes the first and second optical waveguides 111, 112 therein, and are left on the InP substrate 120.
Also, the second i-InP layer 128, the p-InP layer 129, and the p-InGaAsP contact layer 130 are formed as first and second ridge portions 111a, 112a along the first and second optical waveguides 111, 112. Then, the first optical waveguide 111 is constructed by the first ridge portion 111a and respective lower layers. Also the second optical waveguide 112 is constructed by the second ridge portion 112a and respective lower layers.
Multi-layered structures each constructed by laminating the high-resistance InP buffer layer 121, the first InGaAsP etch-stop layer 122, the n+-InP layer 123, the n−-InP layer 124, the MQW layer 125, the first i-InP layer 126, the second InGaAsP etch-stop layer 127, and a third i-InP layer 135 are formed in the slot-line areas on both sides of the area, which includes the first and second optical waveguides 111, 112, away from the layers constituting the first and second optical waveguides 111, 112. The slot-line electrodes 115a, 115b are formed on the third i-InP layers 135 of two multi-layered structures respectively. Also, the phase modulation electrodes 115c, 115d are formed on the p-InGaAsP contact layers 130 of the first and second ridge portions 111a, 112a respectively.
The first phase modulation electrode 115c formed intermittently at an interval on the first ridge portion 111a is connected to the first slot-line electrode 115a via an air-bridge wiring 115e. Also, the second phase modulation electrode 115d formed intermittently at an interval on the second ridge portion 112a is connected to the second slot-line electrode 115b via an air-bridge wiring 115f. In this case, a DC pad 115g is formed on the second InGaAsP etch-stop layer 127.
A sectional structure in FIG. 4 is formed to put the MQW layer 125, which is the core layer of the very thin optical waveguide, between conductive semiconductor layers vertically, and has a large electric capacitance per unit length.
The characteristic impedance of the uniform transmission line constructed by such a layer structure, which has the large electric capacitance per unit length, becomes a vary small value. Therefore, such characteristic impedance is largely deviated from a desired value, e.g., 50 Ω.
In contrast, the characteristic impedance of the slot-line portion only has a relatively large value because an interval between two slot-line electrodes 115a, 115b constituting them is large. Such value can be brought close to a desired value to some extent by adjusting the interval between two slot-line electrodes 115a, 115b. 
Here, the characteristic impedance obtained when the overall device with the addition of such phase modulation area and the slot-line area is regarded as the transmission line of the millimeter wave is derived as an average value of the characteristic impedance of the slot-line area and the characteristic impedance of the phase modulation area.
As a result, though the structures of the phase modulator portions are still maintained as the layer structure having the large electric capacitance in FIG. 4, the characteristic impedance of the overall device can be designed to come close to the desired value by adjusting the structures of the slot-line electrodes 115a, 115b that are combined with the structures of the phase modulator portions. Otherwise, the characteristic impedance of the overall device can be adjusted by adjusting an occupied rate of a total length of the phase modulation electrodes 115c, 115d to a total length of the slot-line electrodes 115a, 115b. 
Also, in the sectional structure in FIG. 4, two optical waveguides 111, 112 constituting the Mach-Zehnder interferometer are formed as the ridge-type optical waveguide, and consists of the MQW layer 125.
Because of the ridge-type optical waveguide, the MQW layer 125 as the core layer spreads laterally. In this optical modulator, the MQW layer 125 is continuous between two arms. The n-InP layers 123, 124 are formed uniformly directly under or below the MQW layer 125, and widths of these n-InP layers are identical to the MQW layer 125 and are formed continuously between two arms. Also, the layer provided directly on the MQW layer 125 is formed by the first i-InP layer 126, and this first i-InP layer 126 is spread to have the same width as the MQW layer 125. The ridge portions 111a, 112a provided over the first i-InP layer 126 are formed by the second i-InP layer 128 and the p-InP layer 129 in order of nearing to the MQW layer 125 respectively. These widths are almost 2.0 μm like the normal optical waveguide.
FIG. 5 is a sectional view taken along a III—III line in FIG. 3 and showing the phase modulator in the direction parallel with the optical axis of the phase modulator.
As shown in FIG. 5, the phase modulation electrodes 115c, 115d by which the electric field is applied to the MQW layer 125 to execute the phase modulation are intermittently formed over two arms constituting the Mach-Zehnder interferometer along the axial direction the arms. Also, the p-InP layer 129 made of the conductive semiconductor and the p-InGaAsP contact layer 130 are formed directly on the MQW layer 125 in the areas of the arms in which the phase modulation electrodes 115c, 115d are not formed.
In the optical modulator in the prior art, there is such a problem that an energy loss generated when the high-frequency electric signal is caused to propagate to the traveling-wave electrode on the device at the time of optically modulating operation becomes large. As a result, there is caused such a problem that a frequency band used when the high-frequency electric signal propagates through the traveling-wave electrode of the device becomes low, and thus the optical modulation band is suppressed low.
In the optical modulator in the prior art, a loss factor generated when the high-frequency electric signal propagates through the traveling-wave electrode on the device becomes α=200 to 250/m. In this case, an intensity of the high-frequency electric signal, which propagates through the traveling-wave electrode having a length L m, attenuates at the rate of exp(−αL). Four causes for such large loss of the high-frequency electric signal as above are considered. These causes will be discussed in detail sequentially hereunder.
First, the first cause will be explained hereunder.
In the sectional layer structure of the optical modulator shown in FIG. 4 in the prior art, the p-InP layers 129 is formed on or over the undoped i-MQW layer 125 and the i-InP layers 126, 128. If such layer structure is formed by the epitaxial growth by virtue of the MOVPE method, Zn acting as the dopant of the p-InP layers 129 is diffused into the lower i-InP layers 126, 128 and into the i-MQW layer 125 to change a part of these i-type layers into the p-type conductivity.
In FIG. 4, the i-InP layer 126 and the i-MQW layer 125 are formed continuously between two arms.
Accordingly, if these layers are changed to have the p-type conductivity, respective phase modulation electrodes 115c, 115d on two arms are connected electrically to each other via the lower p-InP layers 129 and the p-InGaAs layers 130, and these i-type semiconductor layers whose conductivity is changed into the p-type conductivity. Normally, if electrical isolation between two electrodes through which the high-frequency electric signal propagates is incomplete, the loss of the high-frequency electric signal that is caused to propagate between the electrodes is increased. As a result, there is such a problem that the frequency band used when the high-frequency electric signal propagates through the device becomes small and thus the optical modulation band is suppressed small.
It is possible to take such a measure that the diffusion of Zn should be suppressed by forming the i-InP layers 126, 128 formed immediately on or over the MQW layer 125 sufficiently thick. In this case, thicknesses of the i-type layers including the MQW layer 125 become large. If the thicknesses of the i-type layers are increased, the electric field used to modulate the light is applied uniformly to the overall undoped layers and therefore the intensity of the electric field becomes smaller than the case where the overall i-type layers are formed thin. As a result, there is caused such a problem that an increase in a driving voltage is caused.
Then, the second cause will be explained hereunder.
As described above, at the time of modulating operation of the semiconductor optical modulator in the prior art, the high electric field caused by the high-frequency electric signal is applied to concentrate on the i-type layers including the MQW layer 125. Since this electric field exudes into the p-InP layer 129 formed immediately on the i-type layers, such electric field overlaps with the semiconductor that has the electric conductivity in this portion. Such overlapping of the electric field with the conductive material causes the energy loss of the electric signal at the high frequency. This is similarly true of the n-InP layers 123, 124 formed under or below the MQW layer 125. As a result, loss of the high-frequency electric signal is large in the semiconductor optical modulator in the prior art.
Then, the third cause will be explained hereunder.
In FIG. 4, the MQW layer 125 is present continuously between two ridge waveguides. But the electric field, although an amount of such field is minute, is generated by the high-frequency electric signal in the area between two ridge waveguides.
The core consisting of the MQW layer 125 is an undoped layer and is formed as a dielectric layer. In this case, the loss of the high-frequency electric signal is also caused slightly due to the overlapping of the electric field with the MQW layer 125. Also, since a part of the i-MQW layer 125 is changed into the p-type because of the influence of the diffusion of Zn, as described above, a larger loss is caused in this area. These losses act as one of factors of restricting the optical modulation bandwidth.
Then, the fourth cause will be explained hereunder.
As apparent from FIG. 5, the phase modulation electrodes 115c, 115d used to execute the phase modulation by applying the electric field to the MQW layer 125 along the optical axis direction of the arms are formed intermittently in one direction over two arms constituting the Mach-Zehnder interferometer. In this case, the p-InP layer 129 and the p-InGaAsP contact layer 130, which are made of the conductive semiconductor, are also formed directly on the MQW layer 125 in the areas in which the phase modulation electrodes 115c, 115d are not formed on the arms.
In the optical modulating operation, the high-frequency electric signal is transmitted to the phase modulator portion in which the phase modulation electrodes 115c, 115d are formed, and thus the electric field is generated between the phase modulation electrodes 115c, 115d and the n-InP layers 123, 124. At this time, a part of the high-frequency electric signal that transmitted to the phase modulator portions exudes into the axial direction of the arms to generate the electric field in the semiconductor portion in which the phase modulation electrodes 115c, 115d are not provided. Since the exuded electric field overlaps with the semiconductor portion such as the p-InP layer 129, etc., its energy is lost. In other words, such exuded electric field acts as the cause to generate the loss of the high-frequency electric signal and thus acts as the cause to restrict the optical modulation bandwidth, as described up to now.
As described above, in the semiconductor optical modulator in the prior art, the conductive semiconductor layers or the undoped semiconductor layers are formed in various portions on the device. Thus, the loss of the high-frequency electric signal is caused due to the overlapping of these material with the electric field generated by the high-frequency electric signal. As a result, there is such a problem that the optical modulation bandwidth is reduced.
In any case, the cause resides in that the conductive or dielectric semiconductor layers are formed in the areas that are not concerned with the modulation operation. It is desired that the semiconductor layers should be formed at a minimum volume between the metal electrodes in the neighborhood of the MQW core layer in which the phase modulation is executed.